1. Field of the Invention
The present invention relates to a method of on-line coating a coat film on the inner walls of the reaction tubes in a hydrocarbon pyrolysis reactor so that the formation and the deposit of coke on the inner walls can be prevented and inhibited. More particularly, the invention relates to a method of on-line coating an inorganic coat film on the inner walls of the reaction tubes to prevent the formation and the deposit of coke on the inner walls, to remove residual coke continuously, and to inhibit the carburization and deterioration of metals on the inner walls so that the operational duration of the reactor can be extended.
Reactors for use in the pyrolysis of hydrocarbons are commonly composed of a heating furnace and a series of tubular reactors, and are used in producing olefins such as ethylene, propylene and the like, by supplying steam and a hydrocarbon feedstock into the tubular reactors concurrently at an elevated temperature of above 800xc2x0 C. in gaseous phase to induce the hydrocarbon feedstock pyrolysed. During the pyrolysis reaction, coke is formed as a by-product from the dehydrogenation of hydrocarbon by way of the catalytic and/or pyrolytic reaction. Catalytic coke is formed from catalytic reaction between a hydrocarbon and metals such as nickel and iron, which are present on the surface of the tubular reactor. Dehydrogenation of light olefins such as acetylene produces gaseous cokes, and dehydrogenation of heavy aromatic materials yield condensed coke.
These gaseous and condensed cokes are collectively referred to as a pyrolytic coke. As the pyrolysis reaction runs, the coke aggregates and accumulates on the inner walls of the reactor, alone or through a cooperative trap action.
Such aggregation or accumulation of the coke on the inner wall of the reactor tubes interferes with the flow of fluids in the pyrolysis reactor, causing an increase in the pressure drop between the frontal and the postal zones of the reactor, and deteriorates the efficiency of the heat transfer through the inner walls. This results in the reduction of the yields of the main product and the increase of the energy consumption. Carburization may also occur into the metals of which the reaction tubes are fabricated, reducing the durability of the reactor tubes.
2. Disclosure of the Prior Art
Therefore, when the coke is accumulated on the inner walls of the reaction tubes to a certain level, the operation of the reactor must be shut down to eliminate the coke accumulated. The amount of the production loss and the energy consumed to eliminate the accumulated coke are considerably high. Thus, many approaches have been proposed to prevent and inhibit the formation and deposit of the catalytic coke on the inner walls of a hydrocarbon pyrolysis reactor and to extend the cycle of operating the reactor.
Such approaches include: a method of using a specific alloy as a reactor tube material; a method of continuously injecting a certain chemical such as sulfur, an alkaline metal salt, an alkaline earth metal salt, phosphor, boron, cerium, lanthanum, molybdenum, or the like into a hydrocarbon feedstock; a method of pre-treating the inner surfaces of the reaction tubes with tin and silicone, aluminum, and phosphor; a method of allying a ceramic film onto the inner walls of a tubular reactor by molten-coating the inner walls with an alkaline earth metal compound; a method of physical vapor deposition of a mixture of a metal and ceramics on the inner walls of the reaction tubes; a method of chemical vapor deposition of silicone ceramics on the inner walls of the reaction tubes; and so forth.
U.S. Pat. Nos. 4,889,614 and 5,358,626 disclose a process for the gasification of coke into carbon monoxide or carbon dioxide by continuously injecting an alkaline metal or alkaline earth metal salt as a catalyst into a pyrolysis reactor during the pyrolysis of the hydrocarbon. However, this process has a drawback in that a considerable amount of the catalyst may be entrained into and accumulated on the recovery section.
WO 97/41275 discloses a method of forming a protective oxide film by coating a mixture of chromium, aluminum and silicon on fresh reaction tubes in a thickness of about 300 xcexcm by way of a physical vapor deposition and then oxidizing the resultant coat film. This method, however, requires a separate step of coating a mixture of metals on the fresh tubes by an off-line method. Once the film has been worn down, it is impossible to re-coat the tubes without replacing the used tubes with new ones.
U.S. Pat. No. 4,099,990 discloses a method of vapor depositing a silica film having a thickness of about 2 xcexcm onto a metallic reaction tube using tetraethoxysilane as a vapor deposition material and steam or carbon dioxide as a carrier. A test for the pyrolysis of ethane in the resultant tubes demonstrated that the amount of the coke accumulated could be reduced by about 80% at a temperature of 850xc2x0 C. or less as compared with the uncoated tubes, while at a temperature of above 850xc2x0 C., no improvement was obtained. In connection with this method, however, it is necessary to consider the appropriate coating thickness and the vapor deposition conditions for performing the function of the silica film as a barrier and to provide a measure by which the mechanical/thermal strength of the film can be secured. If the silica film is too thin, it does not perform its function as a barrier completely and may be easily worn out by carburization or oxidation during decoking. On the other hand, if the film is too thick, it is apt to peel off or rupture due to the difference in thermal expansion between the matrix metal and the silica film. Further, this patent neither suggests nor teaches clearly how to remove the accumulated coke, even though it is possible to reduce the accumulation of the coke to some extent by covering the tube surfaces with an inactive inorganic oxide film to inhibit the catalytic reaction of the metallic components by which the formation of coke is accelerated.
According to the present invention, it has been discovered that by coating the inner walls of the reaction tubes in a hydrocarbon pyrolysis with an inorganic film to prevent a catalytic reaction between a hydrocarbon feedstock and a metal such as nickel and iron, it is possible to lower the formation and the deposit of coke on the inner wall and inhibit the carburization and deterioration phenomena of the metals due to the coke.
Accordingly, the object of the invention is to provide a method of on-line coating the inner walls of the reaction tubes in a hydrocarbon pyrolysis reactor to prevent the formation and the deposit of coke on the inner walls.
The above object of the invention can be achieved by an on-line method of providing a coat film on the inner wall of a reaction tube in a hydrocarbon pyrolysis reactor, which comprises the steps of vapor depositing a mixed solution of a metal alkoxide and a chromic compound on the inner wall concurrently with introducing a carrier at a flow rate of 1-5000 kg/hr/coil at a temperature of 600-900xc2x0 C. under a pressure of 0-3 kg/cm2 to form a buffer layer on the inner walls; and vapor depositing a metal alkoxide as a diffusion barrier on the buffer layer; and then optionally vapor depositing an alkali metal/alkaline earth metal compound alone or mixed with metal alkoxide as decoking layer to obtain a continuously formed coat film.
In this connection, it is noteworthy to understand that the diffusion barrier can be vapor deposited directly on the inner walls under the same conditions as mentioned above, without providing the buffer layer, and that a decoking layer can then optionally be vapor deposited on the diffusion barrier and/or the buffer layer.